PARTS OF 69 KV SURGE ARRESTERS (LIGHTNING ARRESTER)

Surge arresters may be of the valve or
expulsion type. They are rated not only on their normal voltage
classification in kV, but also on their crest voltage capability in
kV at a standard 1.5 × 40-μs wave (or other specified wave), and
their discharge-current capability in amperes or thousands of amperes
(kA).

For high-voltage application, surge
arresters may consist of a number of unit-value valve arresters
connected in series in one overall unit, as shown in below for a
69-kV arrester.

Cross section of a 69-kV Thyrite
(valve) surge arrester.

Lightning or surge arrester elements
are enclosed in an insulated casing. Under severe operating
conditions, or as a result of multiple operations, the pressure
generated within the casing may rise to the point where pressure
relief ratings are exceeded.

The arrester then may fail, with or
without external flashover, exploding and violently expelling
fragments of the casing as well as the internal components, causing
possible injury to personnel and damage to surrounding structures.
The action represents a race between pressures building up within the
casing and an arcing or flashover outside the casing.

The ‘length’ of the casing of the
arrester limits its ability to vent safely. The use of polymer
insulation for the casing permits puncturing to occur, without the
fragmentation that may accompany breakdown and failure of porcelain.